1. Field of the Invention
The present invention relates to a method and an apparatus for monitoring the composition of a binary (two-component) breathing gas mixture, in particular an air-oxygen mixture, intended for supply by a mechanical breathing aid, such as a patient ventilator or an inhalation anesthetic delivery system.
2. Description of the Prior Art
It is known to provide a mechanical breathing aid that is devised for supplying a mixture of gases, such as an oxygen-enriched air mixture or a mixture of nitrous oxide (N2O) in a breathable gas, to a patient as a breathing gas. For this purpose the breathing aid is provided with an inlet for a first component, such as air or other breathable gas, of the breathing gas mixture. This first component may be drawn into the breathing aid from the atmosphere or may be provided from a pressurized source. The breathing aid also is provided with an inlet for a second component, such as oxygen or N2O, of the binary mixture, typically from a pressurized source, and an outlet for a breathing gas mixture. Automatic flow control valves are often associated with one or both of the inlets and are operable to adjust the gas flow there through dependent on a measured air-oxygen mixture and a desired air-oxygen mixture of the breathing gas. A gas sensor is located in gaseous communication with the mixture to monitor the level of one or the other of the components in the mixture, such as monitoring the oxygen level, and to provide an output indicative thereof. A controller usually is provided and is arranged in operative connection to the sensor and to the valves to vary the flow through the associated inlet in response to the sensor output. The gas sensor is typically a chemical sensor, which has a rather limited lifetime as compared with the expected lifetime of the mechanical breathing aid. Although a type of gas sensor is available that is responsive to the paramagnetic properties of component to be monitored, for example oxygen, which does not have the limited lifetime of the chemical sensor, however, this type of sensor is generally expensive and is sensitive to mechanical vibrations that are present in most mechanical breathing aids.
It is an object of the present invention to provide a method and an apparatus for determining the composition of a binary component breathing gas that avoids or at least alleviates the aforementioned problems associated with known methods and devices of this type.
This object is achieved in accordance with the principles of the present invention in a method and an apparatus for monitoring the composition of a breathing gas containing a first gaseous component and a second gaseous component, wherein a data processor is supplied with an indication of the moisture content of the first gaseous component before mixing with the second gaseous component, and wherein the data processor also is supplied with an indication of the moisture content of the second gaseous component before mixing with the first gaseous component. A moisture content value for the mixture is then measured and supplied to the data processor. Using this input, the data processor determines a value indicative of the amount of one or both of the first gaseous component and the second gaseous component in the mixture.
The amounts of one or both of the two components are determined indirectly through the comparison of levels of humidity in the gaseous components and in the mixture. This allows for a relatively robust and inexpensive humidity sensor to be substituted for the gas sensor conventionally employed.
Moisture may be added to one of the gas components to provide a significant difference in moisture content between the two components and thereby enhance the accuracy of the method.
Preferably, the moisture may be added to achieve a predetermined level of humidity in the selected gas component. The humidity sensor associated with the monitoring of humidity levels in the selected gas component may then be omitted and a value of the predetermined humidity level provided directly.